Maybe another application is using the device. When working on Linux, you can verify this with the following commands:
$ lsof /dev/ttyUSB0
COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME
avrdude 7356 jippie 3u CHR 188,0 0t0 266450197 /dev/ttyUSB0
or
€ fuser /dev/ttyUSB0
/dev/ttyUSB0: 7356
Where of course you have to replace /dev/ttyUSB0 with the device file as mentioned in the error message. In case of this question /dev/ttyACM1, but do notice that the number can change when you unplug the board for a moment and then replug it.
In both results, the 7356 is the PID of the process that is locking the device. In my example I was reading flash from the command line, so the output for your specific situation will be different. To identify the process, you can use:
$ ps -fp 7356
UID PID PPID C STIME TTY TIME CMD
jippie 7698 28116 1 20:34 pts/8 00:00:00 avrdude -patmega1280 -carduino -P/dev/ttyUSB002 -b57600 -D -Uflash:r:/tmp/flash:i
or a more portable command:
$ ps -ef | grep 7356
jippie 7698 28116 1 20:34 pts/8 00:00:00 avrdude -patmega1280 -carduino -P/dev/ttyUSB002 -b57600 -D -Uflash:r:/tmp/flash:i